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Research

Sexual dimorphism of sonic apparatus and extreme intersexual variation of sounds in Ophidion rochei (Ophidiidae): first evidence of a tight relationship between morphology and sound characteristics in Ophidiidae

Abstract

Background

Many Ophidiidae are active in dark environments and display complex sonic apparatus
morphologies. However, sound recordings are scarce and little is known about acoustic
communication in this family. This paper focuses on Ophidion rochei which is known to display an important sexual dimorphism in swimbladder and anterior
skeleton. The aims of this study were to compare the sound producing morphology, and
the resulting sounds in juveniles, females and males of O. rochei.

Results

Males, females, and juveniles possessed different morphotypes. Females and juveniles
contrasted with males because they possessed dramatic differences in morphology of
their sonic muscles, swimbladder, supraoccipital crest, and first vertebrae and associated
ribs. Further, they lacked the ‘rocker bone’ typically found in males. Sounds from
each morphotype were highly divergent. Males generally produced non harmonic, multiple-pulsed
sounds that lasted for several seconds (3.5 ± 1.3 s) with a pulse period of ca. 100 ms. Juvenile and female sounds were recorded for the first time in ophidiids.
Female sounds were harmonic, had shorter pulse period (±3.7 ms), and never exceeded
a few dozen milliseconds (18 ± 11 ms). Moreover, unlike male sounds, female sounds
did not have alternating long and short pulse periods. Juvenile sounds were weaker
but appear to be similar to female sounds.

Conclusions

Although it is not possible to distinguish externally male from female in O. rochei, they show a sonic apparatus and sounds that are dramatically different. This difference
is likely due to their nocturnal habits that may have favored the evolution of internal
secondary sexual characters that help to distinguish males from females and that could
facilitate mate choice by females. Moreover, the comparison of different morphotypes
in this study shows that these morphological differences result from a peramorphosis
that takes place during the development of the gonads.